Amblyopia is a major cause of vision loss in young children (Sachsenweger, 1968). Although amblyopia is thought to reflect alterations in the neuronal properties of Vi, recent work suggests that amblyopes may have higher level deficits, abnormalities in grouping processes and long-range interactions, and previously unsuspected neural plasticity. We propose to test a number of specific hypotheses about the contributions of each of these factors in limiting spatial vision in amblyopic vision, and to assess the limits and mechanisms of neural plasticity in the spatial vision of adults and children with amblyopia. Aim 1. Abnormal long- and short-range interactions in Amblyopia. Spatial interactions are a critical feature of spatial vision, which serve to sharpen perception of form, and enable features to be grouped into forms. Spatial interactions can be both facilitory and inhibitory, and are thought to have their neural basis in lateral interconnections in the visual nervous system (Gilbert, 1998). Several important recent empirical and theoretical developments provide impetus for a new understanding of spatial interactions (including abnormal crowding and grouping) in amblyopia, and to test a novel theory placing the neural abnormality in amblyopia at the level of integrative (grouping) processes. Aim 2. High level deficits in Amblyopia. The main site of the deficit in amblyopia is thought to be the primary visual cortex (Vi). Our recent work suggests that strabismic amblyopes may also have higher level deficits. We propose to explore and assess the generality and implications of this surprising result, to develop a quantitative model for normal and amblyopic counting, and to test several specific hypotheses about the nature and locus of these high level deficits. Aim 3. Neural Plasticity in Amblyopia. Our pilot data suggest a high degree of neural plasticity in some adults with amblyopia. We propose to assess the time course limits and mechanisms of this plasticity in both adults, and young children (three and up) with amblyopia. In particular, our hypothesis is that the improvement that occurs following successful treatment of amblyopia is a result of boosting the brain's ability to extract the relevant information (improved efficiency) by strengthening the weighting of appropriate channels, and reducing input from irrelevant channels (noise exclusion), similar to improvements that occur in normal vision following perceptual learning.